Thin film transistors (TFTs) are pixel-driven components of an active matrix liquid crystal display (AMLCD) and an active matrix organic light emitting diode (AMOLED) display, and play an important role in realizing large-size, high-definition and high-frame-frequency displays. At present, materials of an active layer of a TFT mainly include hydrogenated amorphous silicon, low temperature polycrystalline silicon, organic semiconductors, and oxide semiconductors. Oxide semiconductors have high electron mobility and good uniformity, thus are suitable for driving AMLCDs and AMOLEDs. In order to realize high-resolution display, the TFT needs to be miniaturized, and a realization of a back-channel etched (BCE) structure is the key to miniaturize the TFT.